The Internet of things (IoT) is the inter-networking of physical devices, vehicles, buildings, and other items embedded with electronics, software, sensors, actuators, and network connectivity that enable these objects to collect and exchange data. IoT is expected to offer advanced connectivity of devices, systems, and services that goes beyond machine-to-machine (M2M) communications and covers a variety of protocols, domains, and applications.
IoT can be encapsulated in a wide variety of devices, such as heart monitoring implants; biochip transponders on farm animals; automobiles with built-in sensors; automation of lighting, heating, ventilation, air conditioning (HVAC) systems; and appliances such as washer/dryers, robotic vacuums, air purifiers, ovens or refrigerators/freezers that use Wi-Fi for remote monitoring. Typically, IoT devices encapsulate wireless sensors or a network of such sensors.
Most IoT devices are wireless devices that collect data and transmit such data to a central controller. There are a few requirements to be met to allow widespread deployment of IoT devices. Such requirements include reliable communication links, low energy consumption, and low maintenance costs.
To this aim, IoT devices and wireless sensors are designed to support low power communication protocols, such as Bluetooth low energy (BLE), LoRa, and the like. However, IoT devices utilizing such protocols require a battery, e.g., a coin battery. The reliance on a power source (e.g., a battery) is a limiting factor for electronic devices, due to, for example, cost, size, lack of durability to environmental effects, and frequent replacements.
An alternative to using batteries is power supply which may be harvested from sources such as light, movement, and electromagnetic power such as existing radio frequency transmissions. The harvested power is stored in a capacitor or a rechargeable battery, and typically managed by a power management unit (PMU). A PMU is a circuit block that performs general circuit power related operations, such as supply regulation, voltage and current references, power on indication, brown-out indication, power modes control, management of power storage units, and more.
Specifically, in power harvesting systems, a PMU provides energy storage and voltage threshold crossing indications based on measurement of the voltage over the storage capacitors. Commercially available power harvesting solutions are typically implemented in radio-frequency identification (RFID) and based on a Schmitt trigger.
FIG. 1 shows a diagram of a conventional RFID tag 100 based on a harvester 110. The harvester 110 is coupled to a PMU 120 including a Schmitt trigger 122. The harvester 110 receives RF signals transmitted by a RFID reader (not shown). The energy of the received RF, signals and charges a capacitor 112, where the conversion of energy to current is performed by means of a voltage multiplier 114. A voltage multiplier is an electrical circuit that converts AC electrical power to a DC voltage and cascades its DC outputs to multiply the output voltage level, typically using a network of capacitors and diodes. An example for such a multiplier is a Dickson multiplier.
The PMU 120 determines when the voltage level at the capacitor 112 is sufficient so that the RFID tag 100 can respond to the RFID reader. To this end, a reference voltage threshold (Vref) is compared to the voltage level (Vin) at the capacitor 112. Once the voltage level Vin is over the threshold 121, the Schmitt trigger 122 switches from zero to one, signaling that the RFID tag 110 device can respond to the RFID reader. The response is typically a sequence of predefined bits. In the conventional prior art solutions, the PMU 120 is always based on a single threshold (Vref) and a non-stringent power requirement.
A Schmitt trigger 122 is a comparator circuit with a hysteresis 124 implemented by applying positive feedback to the noninverting input of a comparator or differential amplifier. A Schmitt trigger is an active circuit which converts an analog input signal to a digital output signal. As such, power is required to operate the Schmitt trigger 122. The power is provided by the harvester 110.
Typically, RFID tags provide simple, low power consuming, processing and computing capabilities. Their operations are based on very strong (up to 4 Watts) signals transmitted from RFID readers over the air. The input sensitivity (e.g., lowest operation power received over the air) is about −15 dBm.
The harvester 110 implemented in RFID tags cannot be efficiently utilized to power other low-power communication devices (e.g., BLE devices). The reason is that the BLE protocol is not designed to transmit strong RF signals by external sources. Further, any existing commercial BLE consumes power in a way that RF harvesting is not possible, as the BLE standard has not been designed for battery-powered devices. An attempt to design a harvester based on parasitic RF signals in the air would require a high sensitive harvester, loaded by extremely low power devices. That is: a harvester that operates on very low power RF signals. Further, the power consumption by the harvester and the PMU should be minimized, so that the harvested power would not be completely utilized to power the harvester and the PMU.